1. Field of the Invention
The present invention relates generally to apparatus for automatically guiding a magnetic tape past a rotary head scanner at a predetermined angle other than zero degrees.
2. Description Relative to the Prior Art
In helical scan recorder/playback apparatus, magnetic tape is generally pulled around a cylindrical assembly called a head scanner. The head scanner includes a rotatable headwheel having two or more magnetic heads angularly distributed equally around its periphery. A stationary part of the head scanner may include a machine-shouldered band which guides the tape as it travels around the scanner. Commonly, the oxide side (information-bearing surface) of the tape is in contact with the circumferential surface of the head scanner a little more than one hundred and eighty degrees (180.degree.).
In an embodiment commonly used in the prior art, stationary guide posts are strategically placed where the tape enters and exits the head scanner. These guide posts, commonly called the entrance and exit tape guides, are tilted so that the entrance guide starts changing tape elevation as it travels around the head scanner and the exit guide straightens the tape as it leaves the scanner.
Since the tape is angled as it travels around the head scanner, each magnetic head crosses the width of the tape at a shallow angle. Each pass, or sweep, starts near one edge on the oxide side of the tape and finishes near the opposite edge. Since the recording path of the tape is in the shape of a helix, this arrangement is called helical-scan, or slant-track, recording.
An elaborate and complicated mechanism is commonly employed for threading the tape from a supply reel along its transport path, including the head scanner, to a take-up reel. In one such mechanism, a pair of roller guides, mounted on one end of a pivotal tension arm, extracts tape partially from a cassette. A servomechanism for tape reel drive motors maintains the tape under proper tension. Once the tape is partially extracted, the entrance and exit guide posts, mounted on respective motor driven carriers on opposite sides of the head scanner, serve to engage the non-oxide side of the tape so as to wrap the tape partially around the head scanner at a required helical angle. Before the tape can be actually transported at the proper helical angle, however, additional linkages, in the form of pushrods and levers, are required for changing the respective elevations of the guide posts during movement to their respective final tape-engaging positions. For the threading operation to work precisely, as is required for tracking control by a head scanner, the shafts of the guide posts come into contact with "prisms" of the V-block type required for repeatably locating and locking the posts in their final positions.
Conversely, the unthreading of the tape requires all the previously described movements in respective opposite directions and in reverse sequence. As can be appreciated by those skilled in the art, a tape-threading mechanism of this type is relatively expensive and involves many steps in its manufacture. Also, whether loading or unloading tape, either sequence of operations is relatively time-consuming.
Underlying the tape guide apparatus of the prior art is a general belief that the life of magnetic tape may be extended if the elements of a tape transport path are arranged to provide minimum contact with the oxide side--the information-bearing surface--of the tape. Although it may arguably serve to some extent to extend tape life, a transport arrangement of this type presents a problem in that it is now more difficult to automatically position tape along the transport path since the tape must be placed between transport elements located entirely or primarily on the non-oxide side of the tape, and the transducer head which must be located on the other side.